1. Field of the Invention
The present invention relates to a MIMO (Multiple Input Multiple Output) system of mobile communication and, more particularly to a method for constructing and transmitting feedback information in a closed-loop MIMO system.
2. Description of the Related Art
The MIMO system is a technique for increasing capacity and high data rate transmission.
A closed-loop MIMO system feeding back channel information to a transmitter can acquire larger capacity and a lower bit error rate.
In spite of the advantages of the closed-loop scheme, system designers are reluctant to apply the closed-loop scheme for a physical system because the closed-loop scheme requires the very large feedback data and thus encroaches on one capacity of uplink and downlink.
The reason why the capacity of the feedback data is so large is because the feedback data is a matrix including complex numbers as elements and a dimension of the matrix is the product of the number of transmit antennas and the number of receive antennas.
In order to solve such a problem, a receiver of a related art closed-loop MIMO system feeds back only an index of an optimum solution.
FIG. 1 illustrates the construction of a general closed-loop MIMO system.
As shown in FIG. 1, a receiver 200 of the closed-loop MIMO system obtains Coptimal by using a matrix (H) indicating a wireless channel environment, compares a distance between Coptimal and elements of a predetermined matrix set (referred to as ‘predefined set’, hereinafter), and selects the closest element as a matrix with optimal performance (referred to as ‘optimal solution’, hereinafter).
Equation (1) shown below is an equation of an SVD (Singular Value Decomposition), one of methods used for obtaining Coptimal, in which ‘U’ means Coptimal.HHH=UΣUH  (1)
After the optimal solution is obtained, the receiver 200 feeds back information on the optimal solution to a transmitter 100. Because the transmitter 100 and the receiver 200 share information on the predefined set, the receiver 200 feeds back only the index of the optimal solution.
In FIG. 1, {C1, C2, C3, . . . , CL} defines the predefined set and each element of the predefined set is a matrix. Guides of the matrices {C1, C2, C3, . . . , CL} are orthonormality and have a maximum minimum distance. The orthonormality means CmHCm=I and the maximum minimum distance means that the predefined set should be formed with matrices whose minimum distance is the maximum. In order to determine elements of the predefined set, the receiver 200 searches every matrix and then determines matrices whose minimum distance becomes the maximum as candidate matrices of the predefined set.
However, the afore-mentioned related art has the following problems.
That is, first, in order to form the predefined set only with two guides, numerous orthogonal matrices are to be randomly selected as many as an arbitrary number and matrices whose minimum distance is the maximum are to be picked up, for which, thus, the arbitrary number must be considerably large. Namely, the related art method for forming the predefined set is ineffective because it lacks a structure and regularities.
Second, the processes are to be repeatedly performed whenever the number of transmit antennas is changed, making it difficult to immediately cope with a change in the system.
Third, elements of the predefined set are determined by searching every matrix one by one, which is very ineffective.